1. Field of the Invention
The present invention relates to a diffuse reflection type photochromic element, and relates more particularly to a diffuse reflection type photochromic element in which a transparent state and a diffusely reflective state which diffuses light can be controlled at will. In light of the fact that in the technical field of photochromic glass, in which as a photochromic material the trasmissivity of light of which can be reversibly altered, conventional photochromic glass has suffered from the problem of reflecting sunlight and the like too glaringly when in a clear mirror-like state for example, the present invention provides a novel diffuse reflection type photochromic element capable of providing a new diffuse reflection type photochromic glass which does not suffer from this problem. The present invention is useful in that it provides a novel diffuse reflection type photochromic material to be used in photochromic glass for automatically controlling sunlight entering through window glass without the use of blinds or curtains, a diffuse reflection type photochromic glass prepared using this material, a window material for controlling the transmittance of sunlight in buildings and the like.
2. Description of the Related Art
In buildings in general, windows (openings) are a major source of heat loss and gain. For example, the proportion of heat which escapes through the windows when a building is heated in the winter is about 48%, while the proportion of heat entering through the windows when a building is cooled in the summer is about 71%. Consequently, great energy-saving benefits can be obtained by successfully controlling light and heat at the windows. Photochromic glass was developed for this purpose and serves the function of controlling the gain and loss of heat and light.
There are various systems for performing light control of such photochromic glass. These include 1) materials the transmissivity of which is reversibly altered by application of current or voltage (called as electrochromic materials) and 2) materials the transmissivity of which is reversibly altered by controlling atmospheric gas (called as gasochromic materials). Of these, research is more advance in the field of electrochromic light control glass using a thin film of tungsten oxide for the light control layer, a technology which is almost at the practical stage, with products already on the market.
Known electrochromic light control glasses, including tungsten oxide glass, are all based on the principle of controlling light by absorbing light at the photochromic layer. In this case, the problem is that this kind of photochromic glass acquires heat when light is absorbed by the photochromic layer, and this heat is radiated back into the room, detracting from the energy-saving benefits. To avoid this problem, light needs to be controlled by reflecting it rather than by absorbing it. There is demand for materials which have the property of reversibly changing from a mirror state to a transparent state.
For a long time no such photochromic material could be found capable of reversibly changing from a mirror state to a transparent state, but in 1996 a Dutch group discovered that hydrides of rare earths such as yttrium and lanthanum could be switched from a mirror state to a transparent state by means of hydrogen, and such materials were named “switchable mirror” (J. N. Huiberts, R. Griessen, J. H. Rector, R. J. Wijngaarden, J. P. Dekker, D. G. de Groot and N. J. Koeman, Nature 380 (1996) 231). These rare earth hydrides vary greatly in transmissivity and have excellent switchable mirror properties. However, because rare earth elements are used as materials in these switchable mirrors, there are resource and cost problems when they are used as window coatings and the like.
Next, in 2001, a U.S. group discovered the magnesium-nickel alloy Mg2Ni as a new switchable mirror material (T. J. Richardson, J. L. Slack, R. D. Armitage, R. Kostecki, B. Farangis and M. D. Rubin, Appl. Phys. Lett. 78 (2001) 3047). The elements used in this material are magnesium and nickel, which are cheaper and easier to obtain than rare earth elements. Consequently, this material is expected to be better suited to window coatings. However, although this material is highly reflective when in a mirror state, its optical transmissivity in a transparent state is low (20% according to the literature), and this transmissivity would need to be improved in order for the material to be practical.
After work at improving the switchable mirror characteristics of this magnesium-nickel alloy thin film, the present inventors have discovered that a magnesium-rich magnesium-nickel alloy thin film has good switchable properties, and developed a switchable mirror glass using a magnesium-nickel alloy thin film (Japanese patent publication No. 2003-335553).
However, when this switchable mirror is used in a building, the problem is that if the mirror state is too clear, sunlight and the like will be reflected too glaringly. The recent trend has been to avoid the use in buildings of highly reflective glass which appears mirror-like. Sometimes called “light pollution,” this problem of glass reflection is a problem in the architectural field.